1. Field of the Invention
The present invention relates to a multilayer identity card comprising at least one card surface made of plastic which has in part of its area characters or symbols in a relief structure usable as a printing block, and to a method of producing such identity cards.
2. Discussion of Known Art
Identity cards in the form of credit cards, bank cards, cash cards, entitlement cards or the like are used in a great variety of areas, for example cashless transfers, admittance control systems and a great variety of service systems. These identity cards generally have data related to the card owner which are applied during the so-called "personalization process." A widespread form of representing these characters is embossing, by which the user-related data are embossed from the back of the identity card to the front of the card in relief. To make the individual characters more visible, one dyes them in the peak areas additionally.
However, this financially advantageous form of personalization has the disadvantage that the user-related data are relatively easy to forge. Since the card materials are generally thermoplastics, in particular PVC, the embossed data can be ironed flat or embossed back relatively easily by a manipulator and the cards reembossed with other data. The original dye can be removed using commercially available solvents without any great difficulty and a forged card redyed with a different set of data. It has also turned out that this dye gradually rubs off during daily use. This not only further facilitates forgoing but also often greatly restricts the readability of the data.
In spite of these considerable shortcomings, this form of personalization is used quite widely, in particular for credit cards, since the character set usable as a printing block allows the data to be transferred easily from the card to the current voucher. This is done using so-called "imprinters" into which the card and the voucher are inserted. Via a mechanical embossing process the user data are transferred to the voucher using carbon paper or flimsy (German patent no. 20 18 927). An international standard lays down, among other things, the position of the embossed data on the indentity card and their form and relief height (ISO Standards 7811/1 and 7811/3).
In particular because of the high risk of forgery with these embossed data, proposals have been made for protecting the embossed characters from being changed by taking additional steps (U.S. Pat. Nos. 4,597,592; 4,672,891; 4,597,593 and 4,748,452).
Other developments have attemped to use other techniques to produce characters which are also usable as a printing block but cannot be simply forged.
For example, German laid-open print no. 22 23 290 discloses a method by which the embossed data appear only on one card surface. This method involves embossing the personalization data into metal plates and transferring them to the card surface during lamination of the indentity card. During the laminating process the card material softens and flows into the depressions in the metal plate without leaving a negative relief on the back. This gives rise to relief embossing without any possibility of embossing back the characters.
However, this method is much more elaborate than the usual embossing method since a metal plate with the appropriately engraved personalization data must be produced for each card and this personalization process is a method step that cannot be separated from the card production.
U.S. Pat. No. 4,507,346 was the first to present an identity card and a method of making it by which identity cards can not only be provided in a simple way with characters usable as a printing block, but these charaters are also present in an especially forgery-proof form. This method proposes providing the indentity card with a foamable plastic layer containing appropriate foaming agents. This layer is then formed locally to form the desired characters in a relief structure. A laser beam is preferably used for this purpose which generates in this foamable plastic material the heat necessary for triggering the foaming process or activating the foaming agents. By appropriate selection of the plastic material, the foaming agent and the laser parameters, such as intensity, etc., this foaming process can be performed selectively to produce characters that conform with the standards in terms of their dimensions and their relief height. At the same time this foaming can involve a dyeing of the plastic material, so that the previously necessary dyeing process can be omitted. Since the dyeing takes place in the plastic material itself and is also irreversible, it is forgery-proof and resistant to abrasion.
In order to obtain the relief height of 0.48 or 0.46 mm above the card surface as required by the standard (ISO Standard 7811/1), one must use special plastic films mixed with appropriate foaming agents. Although plastics and foaming agents suitable for this purpose are known, the necessity of incorporating such plastic layers can in some cases means that certain card structures required for other reasons cannot be realized or, for example, the desired transparency is not obtained due to the foaming agents' own color.
The invention is therefore based on the problem of providing an identity card that allows for the simple but forgery-proof individual mode of writing in particular by the laser method, but which can also be provided with a standard embossed character set without using special foamable films.